Trying to work through the README, can't find crate

[saethlin]

╭ ➜ kimockb@gator3:~/kernel
╰ ➤ cat Xargo.toml 
[dependencies.core]
git = "https://github.com/japaric/core64"
╭ ➜ kimockb@gator3:~/kernel
╰ ➤ xargo rustc --target nvptx64-nvidia-cuda -- --emit=asm
   Compiling kernel v0.1.0 (file:///home/kimockb/kernel)
'bdver1' is not a recognized processor for this target (ignoring processor)
'bdver1' is not a recognized processor for this target (ignoring processor)
'bdver1' is not a recognized processor for this target (ignoring processor)
'bdver1' is not a recognized processor for this target (ignoring processor)
'bdver1' is not a recognized processor for this target (ignoring processor)
'bdver1' is not a recognized processor for this target (ignoring processor)
error[E0463]: can't find crate for `core`
  |
  = note: the `nvptx64-nvidia-cuda` target may not be installed

error: aborting due to previous error

error: Could not compile `kernel`.

What am I doing wrong here? It almost seems like xargo isn't picking up the crate URL

[japaric]

You are probably not using a nightly toolchain (check rustc -V). Xargo only works with a nightly toolchain.